See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: The acidity of methylenic (CH2) hydrogens depends on the stability of the carbanion formed after deprotonation. Greater stabilization of the conjugate base (carbanion) through resonance and inductive effects leads to higher acidity. Step 1: Identify the methylenic (CH2) group in each compound and the electron-withdrawing groups (EWGs) flanking it. (a) CH3COCH2CH3 — the CH2 is flanked by one ketone carbonyl (C=O) on one side and a methyl group on the other. Only one EWG stabilizes the carbanion. (b) CH3CH2COOC2H5 — the CH2 is adjacent to one ester group on one side and an ethyl group on the other. Only one EWG (ester) stabilizes the carbanion. (c) CH3CH2CH(COOC2H5)2 — the CH is flanked by two ester groups (malonic ester derivative). Two ester EWGs stabilize the carbanion via resonance, but esters are weaker EWGs than ketones or nitriles. (d) CH3COCH2CN — the CH2 is flanked by a ketone carbonyl (C=O) on one side and a nitrile group (CN) on the other. Both are strong EWGs that stabilize the carbanion through resonance and inductive effects. Step 2: Compare stabilization. - A nitrile group (CN) is a stronger electron-withdrawing group than an ester because it withdraws electrons both inductively and through resonance (pi-system overlap with the carbanion lone pair). - A ketone carbonyl is also a strong EWG stabilizing the carbanion by resonance. - Having both a ketone AND a nitrile flanking the CH2 (as in option d) provides the maximum cumulative stabilization of the resulting carbanion. - Option (c) has two ester groups but esters are less effective at stabilizing a carbanion compared to the ketone+nitrile combination in (d). - Options (a) and (b) each have only one EWG adjacent to the methylenic carbon. Step 3: Rank acidity. The order of acidity: (d) > (c) > (a) > (b), because CH3COCH2CN has two strong EWGs (ketone + nitrile) giving maximum carbanion stabilization. Why other options fail: - (a) Only one EWG (ketone); less stabilization than (d). - (b) Only one EWG (ester), which is weaker than a ketone; least acidic. - (c) Two EWGs but both are esters, which are weaker than the ketone+nitrile combination in (d). Therefore, the correct answer is D.